This invention relates generally to the fabrication of semiconductor devices and in particular to the fabrication of field effect transistors such as amorphous silicon inverted thin film transistors (TFTs).
Thin film transistors fabricated from, among other materials, amorphous silicon, are commonly used to control arrays of solid state devices such as photodiodes, liquid crystal devices, or the like, which form the active parts of displays, facsimile devices, or imagers. Performance of such an array of active devices is typically enhanced by improving response (or switching) time of the TFTs, reducing switching transients and noise, reducing RC response time, and increasing the fill factor (i.e., increasing the portion of the total pixel area taken up by the active device in imager and display arrays).
TFT performance is affected by the mobility of the charge carriers in the semiconductor material of the TFT. High carrier mobilities are desirable as they allow a semiconductor device such as a thin film transistor (TFT) to have high operating speed and high transconductance. Such characteristics result in lower capacitance noise and switching transients, and higher switching speeds, all of which enable small size TFTs to be used in large area displays or imagers, thereby providing more space for the active components of those arrays (such as photodiodes, liquid crystal devices, or the like).
It is thus desirable to fabricate TFTs through a process that produces a transistor having desirable electrical characteristics, such as high carrier mobility. The fabrication process must also produce a device having mechanical characteristics, such as structural integrity and component arrangement, that enable it to exhibit the desired electrical characteristics when put to its intended use. For example, the device must not structurally deteriorate through loss of adhesion between the component layers of the device. Plasma-enhanced chemical vapor (PECVD) deposition is commonly used for depositing materials to form TFTs. The fabrication process necessarily involves deposition of layers of conductive, semiconductive, and insulative material over a substrate and patterning portions of these layers to form the desired TFT island structures. The layers of materials should adhere well to one another during and after being exposed to these processing steps. It is desirable that any treatment to enhance structural stability be compatible with PECVD techniques, which require the device to be exposed to elevated temperatures and electric fields, and reduced pressures during the deposition processes.
Treatment of portions of certain non-TFT semiconductor devices with a hydrogen plasma has been studied. For example, hydrogen plasma treatment applied after the deposition of each thin layer of a superlattice structure was discussed in an article by M. Yamaguchi, K Yatabe, H. Ohta, and K. Morigaki entitled "The effect of hydrogen plasma on the properties of a-Si:H/a-Si.sub.1-x N.sub.x :H superlattices" in Philosophical Magazine Letters, Vol. 58, pp 213-18 (1988). The superlattice structure was formed from alternating layers of amorphous silicon and either semiconductive silicon nitride or insulative silicon nitride. Each of the layers in the Yamaguchi superlattice are much thinner than corresponding layers of similar materials in a typical field effect transistor (FET); e.g., the amorphous silicon layer in the Yamaguchi device has a thickness in the range between 20 .ANG. and 30 .ANG. (as opposed to typical thicknesses of 200 .ANG. to 1000 .ANG. in a FET), and each of the silicon nitride layers has a thickness of about 40 .ANG. (as opposed to thicknesses of 200 .ANG. to 3000 .ANG. in a TFT). Yamaguchi et al. measured photoluminescent spectra, a bulk material phenomenon (as opposed to a phenomenon associated only with the interface of two materials), both in superlattice structures formed with amorphous silicon and semiconducting silicon nitride (a-Si.sub.0.6 N.sub.0.4 :H) layers or with insulating silicon nitride (a-Si.sub.0.43 N.sub.0.57 :H) layers. The Yamaguchi et al. data indicate that the photoluminescent spectra for hydrogen plasma treated samples shifted slightly to peak at a higher photon energy and appeared to not evidence a secondary luminescence peak associated with defects in amorphous silicon. Luminescence intensity also appeared to increase slightly for samples treated with the hydrogen plasma.
It is thus an object of this invention to provide a method of fabricating a thin film transistor that consistently produces a TFT with relatively high mobility in the semiconductor layer which is readily integrated with other TFT fabrication steps.
It is a still further object of this invention to provide a method of fabricating a TFT that exhibits good adhesion between layers of the device.